This invention is related to a wavelength division multiplexing (WDM) transmitters receiver module, more particularly, is related to a WDM transmitter/receiver module that utilizes a micromachined optical mechanical modulator as its transmitter.
Fiber optic communication, particularly fiber-to-the-home (FTTH), drives the demand for low cost, highly reliable transmitter/receiver modules which fulfill standards of successfully implemented technologies such as WDM. At least two types of WDM transmitter/receiver modules have been developed. One type needs an additional semiconductor laser resident at the home and used as the transmitter for the home user. The laser can be directly modulated up to several G bit/s, but is expensive, temperature-sensitive, and power hungry, driving up system cost and affecting performance. The other type is a ring-back architecture that features a single laser in the system, located at the central office, while the home terminus only requires a micromachined optical mechanical modulator. Data to the home is sent in the conventional way; while the data to the office is produced through modulating the light beam from the central office using the micromachined optical mechanical modulator and then sending it back.
For many applications the data to the office may be much slower than the data to the home, i.e., several M bit/s would be sufficient. Such a slow data can be achieved using micromachined optical mechanical modulators. In addition, the micromachined optical mechanical modulator presents the advantage of low cost and good optical performance including low insertion loss, high contrast and polarization insensitivity.
Research on machined optical modulators goes back several decades. These early mechanical optical modulators are of the phase shifting or scanning mirror type. In recent years micromachined optical mechanical modulators based on the interference effect of Fabry-Perot cavity have gained great attention. Microfabrication technologies allow an entire system of electronics integrated in a silicon chip, a few millimeters in size, which are mass-produced from wafers of single crystalline silicon. The same basic fabrication concept and materials, is now being adopted to make low cost, small size, high performance optical components and systems.
Several micromachined optical mechanical modulators have been reported. FIG. 1A and FIG. 1B show a micromachined optical mechanical modulator consisting of a single crystalline silicon substrate 101, an Aluminum supporting frame 102, an air gap 103, and a silicon nitride membrane 104. The silicon nitride membrane 104 is defined as the area released from the single crystalline silicon substrate 101 and consists of the central plate 106 suspended by thin support beams 107. An opening in the electrode material on the central plate 106 of the device defines an optical window 108. To operate the device a voltage is applied to the device through a top electrode 105 and a bottom electrode 109. When the voltage is zero, the gap 103 between the membrane 104 and the substrate 101 is mxcex/4 (m is odd number) and the reflectivity of the Fabry-Perot cavity comes to maximum. When a voltage is increased so that the membrane 104 is deformed downward to the substrate 101 and the gap 103 is (mxe2x88x921)xcex/4, the reflectivity of the Fabry-Perot cavity comes to minimum. In this way an incident light beam can be reflected by the Fabry-Perot cavity and the intensity of the reflected light beam can be modulated through the voltage applied to the device.
The Aluminum/silicon nitride structure is convenient but does present some problems. When deposited thickness approaches 1 xcexcm, the surface of the Aluminum layer can become quite rough and hillocks. Coupled with the surface of the Aluminum layer, the pinhole density of the silicon nitride becomes a greater issue. The existence of pinholes in the silicon nitride or Aluminum hillocks under the silicon nitride can lead to shorting of the electrodes and failure of the device. In addition, as with any micro-machining process, etch selectivity is a concern. This is especially true in forming the Fabry-Perot cavity, since any change in the dimension of the cavity would change the optical properties of the device. Unfortunately, it is impossible to control the lateral dimension of the cavity using an Aluminum layer as the sacrificial material.
In an alternate prior art design, as shown in FIG. 2, the Fabry-Perot cavity consists of a deformable mirror 205, an air gap 206, and a fixed mirror 203 formed on the surface of a single crystalline silicon substrate 201 coated with a silicon dioxide layer 202. The fixed mirror 203 comprises a polysilicon layer formed by low-pressure chemical vapor deposition (LPCVD) and an anti-reflection layer 204. The anti-reflection layer 204 consists of a wet silicon dioxide layer and a LPCVD silicon nitride layer. A 1.6 xcexcm thick phosphorus-doped oxide (PSG) layer is deposited on the surface of the fixed mirror used as a sacrificial layer. A second polysilicon layer is deposited on the surface of the PSG layer. The air gap 206 is formed using selective etching of the PSG layer. A hole 207 with a slanted wall is formed on the backside of the substrate 201. An optical fiber consisting of grading layer 209 and core 208 is inserted into the hole 207 so that the core 208 is aligned with a small hole under the fixed mirror 203.
There are several problems with this design: (1) it is difficult to make a stress free material, or a low tensile stress material for the use of Fabry-Perot cavity. Actually there is a notable compressive stress existed in the polysilicon membrane that makes the membrane uneven and therefore dramatically affects the performance of the Fabry-Perot cavity; (2) the surface roughness of the polysilicon membrane is up to 140 xc3x85 due to the rough surface of the underlying thicker sacrificial PSG layer which results in a higher insertion loss; (3) the applied voltage is up to 70 V due to the thicker sacrificial PSG layer or a wider gap between the two mirrors; (4) the hole for receiving an optical fiber is not precisely aligned with the Fabry-Perot cavity and cannot be used for passive alignment between the Fabry-Perot cavity and the optical fiber; and (5) the Fabry-Perot cavity is out of the plane of the substrate which makes the device easy to break and the production yield difficulty to increase.
It is, therefore, an object of the present invention is to provide a micromachined optical mechanical modulator based transmitter/receiver module, the Fabry-Perot cavity of which is formed by structuring a homo-junction layer stack instead of a hetero-junction layer stack to eliminate any intrinsic stress caused by the mismatch in the thermal conductivity and lattice parameters between the different layers.
Another object of the present invention is to provide a micromachined optical mechanical modulator based transmitter/receiver module, in which the membrane and its supporting beams are finally released by dry etching instead of wet etching to eliminate any stiction caused by the liquid capillary forces occurring during the wet release process.
Still another object of the present invention is to provide a micromachined optical mechanical modulator based transmitter/receiver module, the sacrificial layer of which can be quite thin so that the air gap of the Fabry-Perot cavity formed by removing the sacrificial layer is narrower and therefore the needed operating voltage for changing the distance of the air gap is relatively low.
Still another object of the present invention is to provide a micromachined optical mechanical modulator based transmitter/receiver module, the sacrificial layer of which again can be quite thin, so that the unevenness of the membrane and its supporting beams, which is caused by the rough surface of the thicker sacrificial layer, is relatively low and therefore the insertion loss of the Fabry-Perot cavity is relatively low.
Still another object of the present invention is to provide a micromachined optical mechanical modulator based transmitter/receiver module, the inserting hole of which for incorporating an optical fiber therein can be formed by one-sided processing so that it can be aligned with the central axis of the Fabry-Perot cavity with a high lithographic accuracy.
Still another object of the present invention is to provide a micromachined optical mechanical modulator based transmitter/receiver module, the Fabry-Perot cavity of which has an in-planar configuration so that it can be fabricated using a standard planar semiconductor processing technology and has better mechanical strength leading to a higher production yield.
To realize the above-mentioned objects and other objects, a new micromachined optical mechanical modulator based transmitter/receiver module is provided in accordance with the present invention. As shown in FIG. 3, the Fabry-Perot cavity of the micromachined optical mechanical modulator comprises a top polysilicon membrane and its supporting flexible polysilicon beams 304, a bottom polysilicon membrane 302, and an air gap 306 sandwiched between the top polysilicon membrane and its supporting beams 304 and the bottom polysilicon membrane 302. The central area of the front surface of the top polysilicon membrane is coated with an anti-reflecting layer 305. On the surrounding area of the central area of the front surface of the top polysilicon membrane and the entire area of the top supporting polysilicon beams are coated with an insulating layer 309 and an electrical layer 310. The back surface of the bottom polysilicon membrane 302 is coated with an anti-reflecting layer 303. An electrical contact 308 is connected to the underlying high doping concentration region of a diffusion silicon layer formed in a single crystalline silicon substrate 301. Two metal stripes 313 and 314 are connected to an outside electronic signal source for operating the Fabry-Perot cavity.
The Fabry-Perot cavity is structured from a three-polysilicon-layer stack 307 formed on the surface of the single crystalline silicon substrate. The polysilicon membrane and its supporting polysilicon beams 304 are cut from the top polysilicon layer and released by etching the underlying middle polysilicon layer. Except as a sacrificial layer for releasing the membrane and its supporting beams 304, the middle polysilicon layer is used: (1) as a supporting layer for suspending the membrane and its supporting beams 304; (2) as an insulating layer for electrically isolating the membrane and its supporting beams 304 from the underlying high doping concentration region of the diffusion silicon layer. The bottom polysilicon layer is used as a buffering layer sandwiched between the top polysilicon layer and the underlying single crystalline silicon substrate. The buffering layer can prevent any mismatch stresses from generating in the top polysilicon layer including the membrane and its supporting beams.
A photodiode 316 is mounted over the Fabry-Perot cavity with its solder bumps 318 secured on the surface of the three-polysilicon-layer stack 307. Under the Fabry-Perot cavity there is a conic hole 315 formed in the single crystalline silicon substrate. A rigid plate 312 with a throughout hole is cemented to the single crystalline silicon substrate 301 by applying an adhesive layer 311. An optical fiber 319 is fixed in the conic hole 315 through the throughout hole of the rigid plate 312. The core 320 of the optical fiber 319 is aligned with the central axis of the Fabry-Perot cavity so that the light beam coming out of the core 320 can be precisely projected onto the optical sensitive area 317 of the photodiode 316.
The selective removal of the middle polysilicon layer is done using a porous polysilicon micromachining technology based on selective formation and etching of porous polysilicon. The middle layer of the three-polysilicon-layer stack 307 has a heavily doped polysilicon region surrounded by un-doped polysilicon. The top polysilicon layer has a plurality of openings partially to expose the underlying heavily doped polysilicon region and is used to form the polysilicon membrane and its supporting polysilicon. The bottom polysilicon layer has a plurality of small heavily doped polysilicon regions partially covering the underlying p-type silicon region of the single crystalline silicon substrate. Anodization in hydrofluoric (HF) solution is performed to turn the heavily doped polysilicon into porous polysilicon, while keeping the un-doped polysilicon unchanged. After removing the porous polysilicon in a week silicon etchant, such as a diluted alkali solution the polysilicon membrane and its supporting polysilicon beams are released from the underlying polysilicon layer.
The conic hole 315 of the single crystalline silicon substrate is formed using a porous single crystalline silicon micromachining technology based on selective formation and etching of porous single crystalline silicon. The conic hole 315 is originally filled with porous single crystalline silicon. The porous single crystalline silicon is formed by anodization of the p-type single crystalline silicon in HF solution. In order to form a p-type single crystalline silicon cone, an n-type single crystalline silicon layer ring is formed by thermal diffusion in the p-type single crystalline silicon substrate. Since the thermal diffusion proceeds in both depth and lateral directions the bounding sidewall of the diffusion layer is slantwise. So the p-type single crystalline silicon region surrounded by the n-type diffusion single crystalline silicon layer ring is shaped in an inverse cone with the top diameter larger than the bottom diameter. The p-type single crystalline silicon cone is designed to have a diameter at the top less than the outer diameter of a single-mode fiber so that the cross-section diameter on the halfway of the p-type single crystalline silicon cone matches the outer diameter of the single-mode fiber.
The three-polysilicon-layer stack 307 is formed on the surface of the single crystalline silicon substrate so that its central point is along the central axis of the p-type single crystalline silicon cone.
The anodization is first carried out to turn the heavily doped polysilicon into porous polysilicon. Then the anodization is continued to turn the p-type single crystalline silicon in the p-type single crystalline silicon cone into porous single crystalline silicon.
The polysilicon membrane and its supporting polysilicon beams of the Fabry-Perot cavity are released by using a two-step process. Removing the formed porous polysilicon is the first step for releasing the polysilicon membrane and its supporting polysilicon beams. At this point the polysilicon membrane and its supporting polysilicon beams are still supported by a plurality of un-doped polysilicon poles. The un-doped polysilicon poles are originally surrounded by heavily doped polysilicon. The heavily doped polysilicon is anodized to form porous polysilicon and then removed by etching, while the un-doped polysilicon poles are left behind. The un-doped polysilicon poles are used to prevent the polysilicon membrane and its supporting beams from sticking onto the surface of the bottom polysilicon layer during the etching of the underlying porous polysilicon layer in a diluted alkali solution. As the second step, the un-doped polysilicon poles are removed by dry etching. After the dry etching the polysilicon membrane and its supporting beams are released completely. During the dry etching process no stiction occurs because there is no liquid reactive agent.
In the anodization process both the surface of the polysilicon membrane and its supporting polysilicon beams and the surface of the un-doped polysilicon poles need to be protected. In the dry etching process the surface of the polysilicon membrane and its supporting polysilicon beams still need to be protected, but the surface of the un-doped polysilicon poles does not need to be protected anymore. To meet these requirements, the protecting layer of the polysilicon membrane and its supporting polysilicon beams are comprised of both silicon nitride and silicon dioxide and the protecting layer of the un-doped polysilicon is comprised of only silicon nitride. An etchant for the dry etching is chosen to have higher etch rates for both silicon nitride and polysilicon, lower etch rate for silicon dioxide, and almost zero etch rate for metals, such as a gold and chrome.